The People hub app (contacts list) uses an intelligent merge feature to combine contact information from various sources (e.g. Gmail, Microsoft Exchange, Facebook, etc.).
From the Start screen, swipe left to display the Apps list. Alternatively, tap All apps (located in the lower-right; may require scrolling to the bottom of the Start screen).
Tap People.
Tap a contact.
Tap link (located at the bottom).
Tap choose a contact.
Tap a contact. The contact selected in step 3 is merged with this contact. Repeat steps 5 and 6 as necessary to link additional contacts.